RBSE Prav Voc Automotive cheat sheet

Four-stroke cycle order: intake, compression, power, exhaust; crankshaft completes two revolutions per cycle, camshaft one revolution (2:1 ratio).

Compression ratio = (swept volume + clearance volume) / clearance volume; petrol engines typically 8:1–12:1, diesel 14:1–23:1.

Engine displacement = bore area × stroke × number of cylinders; bore area = π × (bore/2)².

Stoichiometric air-fuel ratio: about 14.7:1 by mass for petrol, 14.5:1 for diesel; lambda = actual AFR / stoichiometric AFR.

Firing order spreads power strokes: common inline-4 orders are 1-3-4-2 and 1-2-4-3; inline-6 commonly 1-5-3-6-2-4.

Valve overlap is the period both intake and exhaust valves are open near TDC; too much overlap at low speed causes rough idle and poor vacuum.

Diesel combustion is compression ignition: injector timing, injection pressure, and glow plugs govern cold starting; petrol uses spark timing and knock sensor feedback.

Torque (Nm) = power (kW) × 9550 / speed (rpm); brake mean effective pressure indicates load independent of displacement.

Cooling system: thermostat opens around 80–90 °C; pressure cap raises boiling point roughly 3 °C per 10 kPa above atmospheric.

Manual gearbox ratio = driven gear teeth / driving gear teeth; overall ratio = gearbox ratio × final drive ratio.

Clutch slip occurs when engine speed exceeds gearbox input speed; free play at pedal spec is typically 10–25 mm before release bearing contact.

Torque converter multiplication occurs at high slip; lock-up clutch engages near cruise to eliminate slip and improve efficiency.

CVT uses two variable-diameter pulleys and a steel belt or chain; ratio change is continuous, so engine speed is decoupled from road speed.

Ackermann geometry: inner front wheel steers a larger angle than the outer so all wheels share a common turning centre.

Caster is the longitudinal tilt of the steering axis; positive caster improves straight-line stability and self-centring.

Camber is wheel tilt from vertical; toe is the difference in distance between front and rear of the tyres measured at hub height.

MacPherson strut combines damper and spring in one unit and provides the upper suspension locating point; double wishbone separates spring and damper functions.

Wheel alignment order: set rear camber and toe first, then front camber, caster, and toe last, with the vehicle at kerb weight.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series circuit: resistances add, current is common; parallel circuit: voltage is common, 1/Rt = 1/R1 + 1/R2 + …

Lead-acid battery: 2.1 V per cell nominal, six cells = 12.6 V at rest; below 12.4 V indicates partial discharge.

Alternator output is rectified three-phase AC; charging voltage typically 13.8–14.5 V at idle with loads off.

Voltage drop test: measure across a conductor under load; more than 0.5 V on a main circuit indicates excessive resistance.

CAN bus is a differential two-wire network (CAN-H and CAN-L) with 120 Ω terminating resistors at each end; parallel resistance should read about 60 Ω.

Relay coil draws low current to switch a high-current contact; a diode across the coil suppresses back-EMF.

Sensor types: NTC coolant sensor resistance falls with temperature; inductive crank sensor generates AC voltage with speed; Hall sensor gives a digital square wave.

Hydraulic pressure = force / area (Pascal's principle); a larger master cylinder bore increases pedal effort but shortens travel.

Brake fluid is hygroscopic; DOT 4 minimum dry boiling point about 230 °C, wet about 155 °C; replace per manufacturer interval.

Disc brake runout and thickness limits are manufacturer-specific; below minimum thickness, heat capacity and pad support are lost.

ABS prevents wheel lock by modulating pressure; wheel speed sensors feed the ECU, and a faulty sensor disables ABS and illuminates the warning lamp.

Electronic stability control compares steering angle with yaw rate and lateral acceleration, then brakes individual wheels to correct the path.

Parking brake must hold the vehicle on a specified gradient; cable free play is adjusted at the equaliser or caliper lever.

Airbag system uses a squib circuit; always disconnect the battery and wait the stated period (commonly 1–10 minutes) before working near airbags.

Brake fade from overheating reduces friction coefficient; recovery requires cooling, and contaminated pads or fluid must be replaced.

Service intervals follow the manufacturer schedule; severe-duty use (short trips, dust, towing) shortens oil and filter intervals.

Engine oil grade: first number is cold (W) viscosity, second is hot viscosity at 100 °C; follow the API/ACEA specification in the manual.

Tyre tread minimum legal depth is 1.6 mm across the central three-quarters; check pressures cold and adjust for load.

Pre-service road test records symptoms before work; post-service road test verifies repair and checks for new faults.

Torque wrench use: set to spec, apply smooth force, and re-calibrate periodically; torque-to-yield bolts are single-use.

Fluid checks include engine oil, coolant, brake fluid, power steering, transmission, and washer; note level and condition, not just quantity.

Battery test sequence: visual inspection, state of charge, then load or conductance test; charge before testing if below 12.4 V.

Inspection checklist covers lights, wipers, horn, seatbelts, exhaust, suspension play, brake condition, and fluid leaks.

Micrometer reads to 0.01 mm; vernier caliper typically 0.02 mm; dial gauge (DTI) measures runout and backlash.

Torque = force × perpendicular distance; a torque wrench extension at 90° does not change applied torque, but an in-line adapter does.

Feeler gauges measure valve clearance and gap; plastigauge measures bearing clearance by crushed width.

Compression tester: remove all plugs, hold throttle open, crank a set number of strokes, and compare cylinders within about 10%.

Cylinder leakage tester applies compressed air at TDC; leakage past rings, valves, or head gasket is located by listening and observing the radiator or dipstick.

Multimeter functions: volts DC/AC, ohms, amps (in series), continuity, and diode test; never measure resistance on a live circuit.

Lifting equipment: jack stands on rated points, axle stands on level ground, and wheel chocks; never work under a vehicle supported only by a jack.

Materials: copper crush washers are single-use, thread-locking compound grades differ by strength, and anaerobic sealants cure without air.

COSHH-style principle: substitute, enclose, ventilate, then use PPE as the last control.

Battery charging releases hydrogen; keep flames away, ventilate, and connect the negative terminal last to avoid sparks near the battery.

Fuel and solvent vapour is heavier than air and collects at floor level; use extraction and avoid ignition sources.

Waste segregation: used oil, coolant, brake fluid, solvents, and batteries are separate hazardous streams; never mix or pour to drain.

Fire classes: A solids, B flammable liquids, C gases, D metals, F cooking oils; CO₂ is unsuitable for metal fires.

Compressed air is not for cleaning skin or clothing; limit to 2 bar (30 psi) for cleaning purposes and wear eye protection.

Noise above 85 dB(A) over an 8-hour average requires hearing protection; hand-arm vibration exposure must be limited.

Lockout/tagout: isolate energy, lock, tag, and verify before repair on lifts, compressors, or hybrid high-voltage systems.

Diagnostic sequence: verify the complaint, gather information, check for technical bulletins, test, repair, and confirm the repair.

DTC formats: P (powertrain), B (body), C (chassis), U (network); the first digit indicates generic (0) or manufacturer-specific (1).

Freeze-frame data records conditions at fault detection: engine speed, load, coolant temperature, and vehicle speed.

Fuel trim: positive long-term trim indicates a lean condition (vacuum leak, low fuel pressure); negative indicates rich.

OBD-II readiness monitors must complete before an emissions test; clearing codes resets them and may fail the test.

Oscilloscope use: check injector and ignition waveforms for amplitude, duration, and burn time; compare with a known-good cylinder.

Intermittent faults: use wiggle tests, vibration, heat, and freeze spray to reproduce; record conditions and monitor live data.

Electrical diagnosis: verify power and ground first, then signal; a high-resistance ground causes multiple unrelated symptoms.

Hybrid high-voltage systems commonly operate at 100–650 V DC; orange cabling identifies high-voltage circuits and requires insulated tools and PPE.

Regenerative braking recovers kinetic energy to the battery; friction brakes handle the remainder and may seize from low use.

EV charging: AC Level 1/2 uses the on-board charger; DC fast charging bypasses it and requires battery thermal management.

Lithium-ion battery state of health is capacity fade over cycles; state of charge is the current energy level, not the same as health.

ADAS levels: L0 no automation, L1 driver assistance, L2 partial, L3 conditional, L4 high, L5 full; L2 requires constant driver supervision.

Common rail diesel injection pressures reach 1,800–2,500 bar; injector coding and adaptation are needed after replacement.

Start-stop and mild-hybrid 48 V systems use a belt-starter-generator; the 48 V battery is separate from the 12 V system.

Telematics and OTA updates require network security; a gateway module isolates safety-critical CAN networks from infotainment.

Estimate structure: parts, labour hours × labour rate, consumables, taxes, and contingency; obtain customer authorisation before extra work.

Job card must record customer details, vehicle identification number, odometer, complaint, work performed, parts used, and technician signature.

Warranty terms state duration, coverage, and exclusions; keep parts for inspection and document the repair.

Break-even point = fixed costs / (selling price per job − variable cost per job).

Gross profit margin = (revenue − cost of goods sold) / revenue × 100; labour is usually the highest-margin line.

Communication: explain the fault in plain language, present options with costs, and confirm the customer's choice in writing.

Record keeping: invoices, MOT or inspection certificates, and hazardous waste consignment notes must be retained for the legally required period.

Business planning: market analysis, location, equipment investment, staffing, pricing strategy, and cash-flow forecast.